The shift to optical fiber as the preferred transmission medium in telecommunications has created the need for practical optical switches to perform the routing and distribution functions in the optical network. One category of optical switches is the waveguide spatial switch that selectably transfers an incoming optical signal to a specific one of a plurality of output waveguides, and is controlled by an externally applied electrical control signal without regard to the particular spectral or informational content of that signal. An optical spatial switch generally functions in one of three modes: (1) as a direct coupler switch by splitting single polarization monochromic light and recombining it constructively in the proper channel and destructively in the others; (2) as a gain switch by splitting the light and subsequently absorbing it in the unwanted channels, while amplifying it in the desired channel to compensate for the splitting loss; or (3) as an index switch by either focusing or reflecting the optical energy into the appropriate output channel.
However, a number of factors limit the performance of these different switches. For example, optical switches that depend upon mechanical or thermal mechanisms for switching are too slow for general use in a telecommunications network.
Direct coupler switches are critically dependent on the length of the active interference region for complete switching. They must be fabricated for operation at a particular wavelength and polarization, which eliminates use in a multi-wavelength network, or a network in which the polarization varies. Gain switches suffer from the high power requirements and complexity of the fast optical amplifiers in the output channels, which must be turned on for amplification or off for absorption.
An index switch can be fabricated to reflect or to focus light by changes in the index of refraction, where the index change is accomplished by either injecting or depleting electrical carriers in the switching region. The depletion mode of operation suffers from polarization sensitivity since the depletion region's large electrical field couples to the material's electro-optic coefficients.
Conventional optical index switches require electrodes on each output branch to effect index changes so that a propagation tendency in the higher index branch will result due to asymmetric adiabatic modal evolution. However, these index switches require stringent fabrication procedure and tolerances to minimze any coupling between the electrodes, particularly in the interaction region at the waveguide junction. Moreover, index switches require continuous electrical power to remain in a particular switching state.